Field of the Invention
This invention relates to industrial burners and, more specifically, to regenerative burners which reduce NOx emissions.
Description of Related Art
Conventional industrial burners are typically configured in a symmetrical fashion. In this symmetrical configuration, a fuel conduit is generally disposed axially along a centerline of the burner and combustion air is generally introduced immediately about a periphery of the fuel conduit or, with appropriate air ducting, symmetrically about the fuel conduit and radially spaced therefrom. A prior art example of a symmetrical industrial burner is disclosed by U.S. Pat. No. 3,876,362 to Hirose. The symmetrical or “axial” burner disclosed by the Hirose patent attempts to induce a deflected stream of gas from the burner by providing an air inlet in the burner tile structure. The positive axial mass flux of the air and fuel jets cause a recirculation of the products of combustion from the furnace chamber into the burner tile. This induction of the products of combustion into the burner tile and subsequent entrainment into the fuel and air streams causes lower flame temperatures and lower NOx production rates.
However, a burner that could obtain NOx levels even lower than those disclosed in the Hirose patent was needed especially with respect to regenerative burners.
U.S. Pat. No. 6,471,508 to Finke at al., the entire contents of which are incorporated herein by reference, discloses a burner that uses non-symmetrical combustion to obtain NOx levels lower than those disclosed in the Hirose patent. In the disclosed burner, the fuel conduit is coaxial with a line spaced from the central axis of the burner. The air conduit is placed on the opposite side of the burner central axis from the fuel conduit. This configuration demonstrates a reduction in NOx levels over those disclosed by Hirose when used in a recuperative burner having air preheat temperatures of around 800° F. (see Table 1). However, when this configuration is used in a regenerative burner where the air may be preheated to >2000° F., the NOx levels, while reduced, are not reduced to the same low levels as those experienced with the recuperative burner. This is partially due to the fact that the low density highly preheated air requires the opening for the conduit for combustion air (passage 36 in the Finke patent) to be enlarged to pass the needed air flow at reasonable pressures, as compared to the lower air temperature recuperative burner. This larger opening is necessarily spaced closer to the fuel opening and, therefore, allows the air flow to mix more quickly with the fuel which tends to increase NOx.
Thus, an improvement to the burner disclosed in the Finke patent is needed to allow lower NOx levels to be achieved in a non-symmetrical regenerative burner.